Use of GPS Approaches in Alternate Calculations
- Dan Boedigheimer
The limiting factor in using an airport with only GPS approaches in your alternate determination was the Aeronautical Information Manual (AIM) section 1−1−19 - Global Positioning System (GPS). “Any required alternate airport must have an approved instrument approach procedure other than GPS that is anticipated to be operational and available at the estimated time of arrival, and which the aircraft is equipped to fly.”
For airports with both ground-based navaid IAPs and GPS-based IAPs, the GPS-based IAPs typically would show as NA for filing as an alternate.
The NOTAM now allows an option to flight plan for use of a GPS-based IAP at either the destination or the alternate airport, but not at both locations.
What if I don’t have WAAS-approved avionics?
At the alternate airport, pilots may plan for applicable alternate airport weather minimums using the following:
- Lateral navigation (LNAV) or circling minimum descent altitude (MDA).
- LNAV / vertical navigation (LNAV/VNAV) decision altitude (DA) if equipped with and using approved barometric vertical navigation (baro-VNAV);
- RNP 0.3 DA on an RNAV (RNP) IAP if specifically authorized with approved baro-VNAV equipment.
To take advantage of this option, GPS users must:
- Have navigation systems with fault detection and exclusion (FDE) capability.
- Perform a preflight Receiver Autonomous Integrity Monitoring (RAIM) prediction for the approach integrity at the airport where the GPS-based IAP will be flown.
- Have proper knowledge and any required training and/or approval to conduct a GPS-based IAP.
- Ensure that the conventional approach (at destination or alternate) can be flown without reliance on GPS.
Following is an example used in the NOTAM:
A GPS user without approved baro-VNAV capability is planning a flight to Frederick, MD (KFDK) with York, PA (KTHV) as the planned alternate. Frederick has an ILS IAP available while York has an RNAV (GPS) IAP with LNAV minimums and an NDB IAP.
Under the revised policy, the GPS user may flight plan for the ILS at Frederick and the RNAV (GPS) LNAV minimums at York for the alternate. The previous policy would have prevented the GPS user from flight planning the RNAV (GPS) IAP at the alternate.
What if I have WAAS-approved avionics?
The current alternate airport planning policy explicitly prohibits WAAS users from planning to use WAAS vertical guidance at their alternate airport (LNAV minimums prior to this NOTAM).
WAAS users equipped with and using approved baro-VNAV equipment may plan for LNAV/VNAV or RNP 0.3 DA at the alternate airport. WAAS users without baro-VNAV may still plan for LNAV at an alternate airport.
There are some WAAS integrations that use baro-VNAV for vertical guidance. WAAS users should consult their flight manuals for this information. This policy clarification allows properly trained and approved WAAS users equipped with and using approved baro-VNAV equipment to plan for applicable alternate airport weather minimums using the following:
- LNAV/VNAV DA at an alternate airport
- RNP 0.3 DA on an RNAV (RNP) IAP at the alternate airport if specifically authorized
For air carriers with Ops Spec C055 issued after February 2011, there is a provision for the certificate holder to use GPS-based IAPs at an alternate airport if you have WAAS-approved avionics. The flight planning is limited on flying the RNAV (GPS) LNAV minima line, or the minima on a GPS approach procedure or conventional approach procedure with “… or GPS” in the title. Even with the NOTAM, air carriers must still comply with their Ops Spec C055 authorization. Look for your company to update to Ops Spec C055 to allow for use of LNAV/VNAV minimums at the alternate.
Following is an example used in the NOTAM:
A WAAS user that also has approved baro-VNAV capability on the aircraft is planning a flight to Frederick, MD (KFDK) with Leesburg, VA (KJYO) as the planned alternate. Frederick has an RNAV (GPS) IAP with LPV minima while Leesburg has an ILS that is out of service per NOTAM and an RNAV (GPS) IAP.
Under the new policy clarification, this WAAS user may flight plan for the LPV at Frederick and the RNAV (GPS) LNAV/VNAV minima (based on using baroVNAV) at Leesburg for the alternate. Since baro-VNAV is supplying the vertical information, the charted temperature restrictions apply.
To stay up-to-date on changes to RNAV procedures enroll in Advanced Aircrew Academy's online RNAV course.